dynamodb_balance.md

• want to subtract 8 from balance, before that we should check balance is greater than 8.

      DynamoDbClient ddb = DynamoDbClient.builder().region(Region.of("us-east-1"))
  			.credentialsProvider(ProfileCredentialsProvider.create("cats")).build();

  	HashMap<String, AttributeValue> itemKey = new HashMap<>();
  	itemKey.put("pk", AttributeValue.builder().s("1_1234").build());
  	itemKey.put("sk", AttributeValue.builder().s("FME").build());

  	Map<String, String> expressionAttributesNames = new HashMap<>();
  	expressionAttributesNames.put("#kn0", "balance");
  	Map<String, AttributeValue> expressionAttributeValues = new HashMap<>();
  	expressionAttributeValues.put(":kv0", AttributeValue.builder().n("8").build());

  	UpdateItemRequest updateItemRequest = UpdateItemRequest.builder().tableName("future_match_engine-uat")
  			.key(itemKey).updateExpression("SET #kn0 = #kn0 - :kv0").conditionExpression("balance > :kv0")
  			.expressionAttributeNames(expressionAttributesNames)
  			.expressionAttributeValues(expressionAttributeValues).returnValues(ReturnValue.UPDATED_NEW).build();

  	try {
  		UpdateItemResponse updateItemResponse = ddb.updateItem(updateItemRequest);
  		System.out.println(updateItemResponse);
  	} catch (ConditionalCheckFailedException e) {
  		System.out.println("fail.");
  	}